(a) Technical Field of Invention
This invention relates to thermal transfer printing and, in particular, to a thermal transfer printing receiver sheet for use with an associated donor sheet.
(b) Background of the Art
Currently available thermal transsfer printing (TTP) techniques generally involve the generation of an image on a receiver sheet by thermal transfer of an imaging medium from an associated donor sheet. The donor sheet typically comprises a supporting substrate of paper, synthetic paper or a polymeric film material coated with a transfer layer comprising a sublimable dye incorporated in an ink medium usually comprising a wax and/or a polymeric resin binder. The associated receiver sheet usually comprises a supporting substrate, of a similar material, having on a surface thereof a dye-receptive, polymeric receiving layer. When an assembly, comprising a donor and a receiver sheet positioned with the respective transfer and receiving layers in contact, is selectively heated in a patterned area derived, for example--from an information signal, such as a television signal, dye is transferred from the donor sheet to the dye-receptive layer of the receiver sheet to form therein a monochrome image of the specified pattern. By repeating the process with different monochrome dyes, a full coloured image is produced on the receiver sheet.
To facilitate separation of the imaged sheet from the heated assembly, at least one of the transfer layer and receiving layer may be associated with a release medium, such as a silicone oil.
At the printing or transfer stage in a typical TTP operation both the transfer layer and the receiving layer are likely to be in a molten state, and there is a tendency for the donor sheet to become thermally bonded to the receiver sheet. Such bonding may induce wrinkling or even rupture of the donor sheet when separation thereof from the image receiver sheet is attempted. In certain circumstances, total transfer of the dye-containing transfer layer to the receiver sheet may occur, so that the donor sheet is effectively destroyed and portions thereof become firmly adhered to the processed receiver sheet. To avoid such undesirable behaviour, the release medium is required to promote relative movement between the donor sheet and the receiver sheet to permit easy separation of one from the other. However, advancement of the donor sheet, relative to the print-head, in register with the receiver sheet usually depends upon frictional engagement between the donor sheet and the receiver sheet the latter being mounted on a forwardly displaceable roll or platen. Inadequate bonding between the respective sheets tends to result in loss of registration, and the generation of a poorly defined image. The release medium must therefore also promote frictional bonding between the donor and receiver sheets, and is thus required to satisfy two apparently conflicting criteria.
The commercial success of a TTP system depends, inter alia, on the development of an image having adequate intensity, contrast and definition. Optical Density of the image is therefore an important criterion, but unfortunately, the presence of a release medium may inhibit migration of the dye into the receiving layer, thereby reducing the optical density of the resultant image. The problem of inadequate optical density is particularly acute if the release medium is modified in any way such that it constitutes a barrier to migration of dye from the donor to the receiver sheet--for example, when the release medium is subtantially cross-linked. Likewise, inclusion in the release medium of extraneous materials likely further to inhibit dye migration is undesirable.
Although the intense, localised heating required to effect development of a sharp image may be applied by various techniques, including laser beam imaging, a convenient and widely employed technique of thermal printing involves a thermal print-head, for example, of the dot matrix variety in which each dot is represented by an independent heating element (electronically controlled, if desired). A problem associated with such a contact print-head is the deformation of the receiver sheet resulting from pressure of the respective elements on the heated, softened assembly. This deformation manifests itself as a reduction in the surface gloss of the receiver sheet, and is particularly significant in receiver sheets the surface of which is initially smooth and glossy, i.e. of the kind which is in demand in the production of high quality art-work. A further problem associated with pressure deformation is the phenomenon of "strike-through" in which an impression of the image is observed on the rear surface of the receiver sheet, i.e. the free surface of the substrate remote from the receiving layer.
(c) The Prior Art
Various receiver sheets have been proposed for use in TTP processes. For example, EP-A-0133012 discloses a heat transferable sheet having a substrate and an image-receiving layer thereon, a dye-permeable releasing agent, such as silicone oil, being present either in the image-receiving layer or as a release layer on at least part of the image receiving layer. Materials identified for use in the substrate include condenser paper, glassine paper, parchment paper, or a flexible thin sheet of a paper or plastics film (including polyethylene terephthalate) having a high degree of sizing, although the exemplified substrate material is primarily a synthetic paper--believed to be based on a propylene polymer. The thickness of the substrate is ordinarily of the order of 3 to 50 .mu.m. The image-receiving layer may be based on a resin having an ester, urethane, amide, urea, or highly polar linkage.
Related European patent application EP-A-0133011 discloses a heat transferable sheet based on similar substrate and imaging layer materials save that the exposed surface of the receptive layer comprises first and second regions respectively comprising (a) a synthetic resin having a glass transition temperature of from -100.degree. to 20.degree. C. and having a polar group, and (b) a synthetic resin having a glass transition temperature of 40.degree. C. or above. The receptive layer may have a thickness of from 3 to 50 .mu.m when used in conjunction with a substrate layer, or from 60 to 200 .mu.m when used independently.
As hereinbefore described, problems associated with commercially available TTP receiver sheets include inadequate intensity and contrast of the developed image, reduction in gloss of the imaged sheet, strike-through of the image to the rear surface of the sheet, and difficulty in maintaining register during the printing cycle.
We have now devised a receiver sheet for use in a TTP process which overcomes or substantially eliminates the aforementioned defects.